Chris at eSpares says...

This switch identifies the water level in the drum by the air pressure in the expansion chamber on the drum. As water pushes air into the chamber air is passed through a narrow pipe to the switch check there is no blockages in the tube and chamber and remove the hose when empty to reset the empty pressure. If the machine then over fills or if it wont go into spin when empty or even if the heater does not come on then there may be a fault with this switch.